In conventional, there has been proposed various configurations as a stratified scavenging two-cycle engine having an air control valve controlling an air amount of a scavenging lead air. For example, there has been proposed a lead air control apparatus of a stratified scavenging two-cycle engine, as described in Japanese Patent Application Laid-Open (JP-A) No. 2000-328945.
The lead air control apparatus described in JP-A No. 2000-328945 is shown in FIG. 9. As shown FIG. 9, in a stratified scavenging two-cycle engine 40, a pair of scavenging ports 10 are provided in opposing both side surface portions, on an inner wall surface of a cylinder 3 to which a piston 4 is slidably fitted, and a pair of scavenging ports 10 are respectively connected to a crank chamber 11 by scavenging flow paths 12.
A carburetor 42 is attached to an intake port 13 provided in the cylinder 3 via an insulator 41 aiming at a heat insulation, and an intake side of the carburetor 42 is connected to an air cleaner 44. The carburetor 42 is provided with a butterfly type throttle valve 43. The insulator 41 is provided with an intake passage 22 connecting the intake port 13 and the carburetor 42, and an air passage 45 for the lead air.
One side of the air passage 45 for the lead air is connected to the air cleaner 44, and the other end thereof is formed in a fork shape so as to be branched into right and left sides, which are respectively connected to the pair of scavenging ports 10 and the scavenging flow paths 12 via connection pipes 46. A butterfly type air control valve 25 controlling the air amount of the lead air is provided in an upstream side of the branch point of the air passage 45, and is configured such as to work with the throttle valve 43 of the carburetor 42.
Accordingly, it is possible to attach the air control valve 25 within a limited space, it is possible to make an entire length M of the engine short, and it is possible to achieve a compact and light configuration.
However, in the configuration described in JP-A No. 2000-328945, the air control valve is provided in the air passage arranged in the insulator, the air passage is branched into the right and left sides in a downstream portion of the air control valve, and the branched air passages are respectively connected to the pair of scavenging ports and a pair of scavenging flow paths which are provided in the left and right sides of the cylinder of the engine. Accordingly, the configuration of the insulator is complicated, and the length of the insulator is elongated, and a large area product is required. As a result, an outer diameter of the engine is increased.
Further, a configuration for easily forming the air passage can be achieved by forming the air passage formed within the insulator approximately in a linear shape. Accordingly, when the air passage is configured so as to be branched into the right and left sides within the insulator, it becomes complicated to form the branched air passage, and an elbow shape, that is, a shape in which the linear air passages cross, is formed at the branch portion in the air passage. If the air passage is formed in a sharply bent shape at the branch portion, there is a problem that the air flow in the bent portion forms a flow in which a vortex flow is generated by being peeled off from the inner wall of the air passage, whereby an air resistance is increased.